carry of #25231: fix Priority row drag hit testing
Carry of #25231: cmux NEXT keep Priority dragged chats in place.
The original PR persists a dragged row's position. Its exact-head review found that normal AppKit presses target title and preview subviews, so row drag handlers never run. This carry routes row hit testing to the row while preserving action-button targets, and adds a regression assertion.
Original: https://github.com/manaflow-ai/cmux/pull/25231
Validation: git diff --check; focused regression added in SidebarPriorityDragTests (fleet test pending).
Agent session: agent session
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Summary by cubic
Carries over #25231 to cmux NEXT: dragging a Priority row now moves it and keeps it at the dropped place, even while it sits idle. The earlier fix never fired because AppKit presses land on the title and preview subviews, so row hit testing now routes presses to the row while action buttons keep their own targets.
Placed chats hold their dropped slot with the rest ranked around them, and placed idle chats stay past the idle horizon like pinned ones. Marks saved before the placed map existed still load, and a regression suite covers row hit testing, drags up and down, and jitter clicks.